Slide won't retract

Sounds like low battery.
Probably house battery I would guess.

Turn on your headlights and once you push the retract switch if the lights dim it is on the chassis battery. If not it is on the house batteries.
You might try starting the vehicle and charging the house batteries enough to retract the slide. Some coaches you can retract with the engine running and some you cannot but at least the alternator can charge the house batteries.
If none of this works, then you can dig out the owners manual and see what you have to do to manually retract the slide.

If it starts and stops, that is a common symptom of a weak auto-restting breaker.

What/Where is an auto resetting breaker? As I said, my house batteries are new and the problem still exists.

Your National uses the House Batteries. The auto resetting breaker is usually located in the fuse block assbly in the right side storage compartments. But, rule out the Auto reset theory if when it stops and you immediately push the button and it jumps again. Auto resets do NOT reset that fast. Usually 3 to 5 seconds up to 10 seconds. To locate that auto reset, find the storage compartment with that breaker/fuse panel. Have someone operate the slide. When a Auto reset trips, it makes a distinctive "clink" sound. Doug